A Fjellgangr declares a Thrash attack as its charge attack and reaches a unit of models. The little giant makes an attack as per the rules for Thrash, but leaves the charged model as the last one to be resolved in this action. Once the attack is made against the target of the charge, we assume that the charged model is killed. This triggers Slay Movement and the Fjellgangr is allowed to move d3" and go for one more swing with Thrash.

Yes that's correct, but you don't Need to do it in that order necessarily; even if you kill the target model of the Charge with the first attack, you can still complete the thrash attacks on other relevant models and then do the slay-movement (which also benefits from Thrash).

Against low-ARM models he is a realy lawn-mower!
To get more effect:

Give the Fjellgangr an extra action token from the Warlord so he has 6, then with Strongarm Ale plus charge bonus, he will be attacking all enemy models in 2 inches at MEL8 and POW5. If he kills the charged model, then he gets to repeat the Thrash attack for free with his Slay Movement. Then he still has 3 ACT left over, so he can make a third Thrash attack!